1. Field of the Invention
This invention relates to a cross-coil meter including a pair of mutually perpendicular coils for generating a magnetic field and a pointer driven by the magnetic field, and more particularly to a PWM modulation circuit to be used in a meter drive apparatus which drives such a cross-coil meter.
2. Description of the Related Art
A cross-coil meter is currently known which generates a magnetic field by supplying a current through a pair of mutually perpendicular excitation coils and indicates a desired angular bearing by imparting to an angularly movably supported magnet a torque by this magnetic field. This cross-coil meter is widely used in, for example, a speedometer and a tachometer of a motor vehicle.
FIGS. 5 and 6 of the accompanying drawings show the general construction of this conventional cross-coil meter.
As shown in FIG. 5, a pair of excitation coils L.sub.s, L.sub.c disposed so as to be perpendicular to each other generate a magnetic field as a pulse current according to the input quantity such as of a speedometer flows therethrough, and a angularly movable permanent magnet M receives a torque due to the magnetic field generated by the two excitation coils L.sub.s, L.sub.c.
Then a pointer fixed on the permanent magnet M is angularly moved through .theta. from a reference position to indicate a predetermined position of a scale plate on which scale marks represent physical quantity (speed in FIG. 6) to be measured and shows a directive value according to the inputted quantity.
A conventional meter drive apparatus for driving the cross-coil meter to be used in a speedometer such as of a motor vehicle inputs usually a pulse signal varying in frequency according to the inputted quantity such as speed. The conventional meter drive apparatus also uses predetermined periodic reference clock signals to be generated by a reference clock generating means and counts the number of pulses of the reference clock signals, which are generated in a cycle of the pulse signal, to calculate the frequency of the pulse signal.
Further the conventional meter drive apparatus supplies to the two mutually perpendicular excitation coils L.sub.s, L.sub.c a current PWM-modulated based on the calculated frequency.
FIG. 7 shows the PWM waveform.
For supplying to the excitation coils L.sub.s, L.sub.c a current according to the frequency of the input pulse signal, angle indices .theta. corresponding to some input quantity values S are predetermined and these values (S, .theta.) are stored in a ROM. When calculating an angle of index .theta. according to the input quantity value S to be indicated, a parameter is read from the ROM and .theta. is calculated according to a predetermined equation stored in the ROM, whereupon a predetermined operating process is performed to cause a current to flow according to the calculated .theta.. This prior art is exemplified by Japanese Patent application No. HEI 1-332280.
As mentioned above, in the conventional cross-coil meter, since a PWM-modulated current, i.e. a current of FIG. 7, is supplied through the excitation coils L.sub.s, L.sub.c, a magnetic field generated by the excitation coils L.sub.s, L.sub.c will also vary in the same cycle as the cycle of PWM. The mechanical system of the meter angularly moves according to the change of this magnetic field and at that time, if the frequency of the magnetic field change coincides with the resonant frequency of the mechanical system, the mechanical system resonates to generate a buzzing noise.
For preventing this buzzing, the base frequency of the field change should be outside the audible frequency range, i.e. usually higher than 20 kHz. Accordingly a cycle of a PWM signal should be less than 50 .mu.sec (=1/20 kHz). If the resolution of 10 bits (=1024) is to be obtained in this cycle, the number of reference clocks should be 48 nsec (=50 .mu.sec/1024), i.e. 20.5 MHz.
Although it is not impossible to design a CMOS circuit which is operable at a frequency of higher than 20 MHz, it could have been easy by far to design the CMOS circuit of a compact size, if the operating frequency was 6.144 MHz, i.e. lower than 1/3.